1. Field of the Invention:
This invention relates to an air-fuel ratio control apparatus in an electronically controlled engine for a vehicle, and more particularly to an air-fuel ratio control apparatus capable of maintaining accuracy in air-fuel ratio control satisfactorily despite changes over time in the output characteristics of an air-fuel ratio sensor.
2. Description of the Prior Art:
In an air-fuel ratio control apparatus for an electronically controlled engine, the output of an oxygen sensor (hereinafter called "O.sub.2 sensor") is used to generate a feedback signal. The O.sub.2 sensor is of a type used for detecting the air-fuel ratio of a mixture in a combustion chamber of the engine from the oxygen concentration in an exhaust system. It changes the level of its output voltage as it detects approximately the stoichiometric air-fuel ratio. However the output characteristics are changed with the passage of time due to degradation or the like and thereby a problem is encountered that the air-fuel ratio also deviates from the stoichiometric air-fuel ratio. Thus, in the prior art, the amplitude of the air-fuel ratio detecting signal from the O.sub.2 sensor is detected to correct a comparative voltage compared with the air-fuel ratio detecting signal in relation with the amplitude for shaping the air-fuel ratio detecting signal. While this prior art can dispose of the change with the passage of time in air-fuel ratio when the amplitude of the air-fuel ratio detecting signal and rich signal and lean signal of the air-fuel ratio detecting signal are changed over (this changed-over air-fuel ratio is approximately the stoichiometric, air-fuel ratio in the normal air-fuel ratio detecting signal), response time in which the air-fuel ratio detecting signal is changed from the rich signal to the lean signal as the air-fuel ratio changes differs from one in which same is changed reversely. In the prior art, it was difficult to hold the air-fuel ratio at a target value by disposing of the change with the passage of time in the difference between these response times.